Optical fiber with low fictive temperature

ABSTRACT

Optical fibers having low fictive temperature and methods of making such fibers are described. Management of the cooling rate of an optical fiber during fiber draw permits control over the fictive temperature of the fiber. Non-monotonic cooling rates are shown to promote reductions in fiber fictive temperature. The non-monotonic cooling includes slower cooling rates in upstream portions of the process pathway and faster cooling rates in downstream portions of the process pathway. Reduction in fiber fictive temperature is achieved by controlling the ambient temperature of the fiber to slow the cooling rate of the fiber in upstream portions of the process pathway that correspond to the fiber temperature regime in which the fiber viscosity is sufficiently low to permit efficient structural relaxation. Increases in cooling rate in downstream portions of the process pathway permit adjustment of fiber temperature as needed to meet entrance temperature requirements of downstream processing units. Lower fiber fictive temperature and lower fiber attenuation are achieved at faster draw speeds through non-monotonic cooling of fiber temperature.

This application is a divisional of and claims the benefit of priorityunder 35 U.S.C. § 120 of U.S. application Ser. No. 15/710,074 filed onSep. 20, 2017, which claims the benefit of priority under 35 U.S.C. §119 of U.S. Provisional Application Ser. No. 62/428,739 filed on Dec. 1,2016 and U.S. Provisional Application Ser. No. 62/404,345 filed on Oct.5, 2016, the contents of which are relied upon and incorporated hereinby reference in their entirety.

FIELD

This description pertains to optical fibers. More particularly, thisdescription pertains to optical fibers having low fictive temperature.Fibers with low fictive temperature exhibit low attenuation of opticalsignals. This description also pertains to methods of controlled coolingto form optical fibers having low fictive temperature.

BACKGROUND

In the manufacturing of the optical fibers, the optical preforms areheated to temperatures well above the glass softening point and thendrawn at large draw down ratios to form optical fibers 125 μm indiameter. Due to the high draw temperatures, large draw down ratios andfast draw speeds, the glass is far from an equilibrium state, resultingin optical fibers with high fictive temperature. High fictivetemperature is undesirable because high fictive temperature is known tocorrelate with increased attenuation of optical signals in opticalfibers. To reduce attenuation in optical fibers, it is desirable tomodify processing conditions to produce optical fibers with lowerfictive temperature.

Efforts to reduce fictive temperature have emphasized slow cooling tostabilize the optical fiber in a state closer to an equilibrium state.Prolonged cooling an optical fiber at temperatures in the glasstransition range of the fiber is one strategy for reducing fictivetemperature. The extent to which fictive temperature can be reduced inexisting fiber processing systems at the draw speeds used in opticalfiber manufacturing, however, is limited because the residence time ofthe optical fiber at temperatures within the glass transition range aretoo short (typically <0.2 sec) to permit significant relaxation of thestructure of the glass. Because of the short residence time, thestructure of the glass remains far from the equilibrium state and theattenuation of the optical fiber remains too high for many applications.It would be desirable to develop methods of processing that enable theproduction of optical fibers having low fictive temperature so thatoptical fibers with reduced attenuation can be achieved.

SUMMARY

Optical fibers having low fictive temperature and methods of making suchfibers are described. Management of the cooling rate of an optical fiberduring fiber draw permits control over the fictive temperature of thefiber. Non-monotonic cooling rates are shown to promote reductions infiber fictive temperature. The non-monotonic cooling includes slowercooling rates in upstream portions of the process pathway and fastercooling rates in downstream portions of the process pathway. Reductionin fiber fictive temperature is achieved by controlling the ambienttemperature in the vicinity of the fiber to slow the cooling rate of thefiber in upstream portions of the process pathway that correspond to thefiber temperature regime in which the fiber viscosity is sufficientlylow to permit efficient structural relaxation. Increases in cooling ratein downstream portions of the process pathway permit adjustment of fibertemperature as needed to meet entrance temperature requirements ofdownstream processing units. Lower fiber fictive temperature and lowerfiber attenuation are achieved at faster draw speeds throughnon-monotonic cooling of optical fibers.

The present disclosure extends to:

A method of processing an optical fiber comprising:

cooling an optical fiber from a first fiber temperature to a secondfiber temperature at a first cooling rate, said first cooling rate beingless than 5000° C./s;

cooling said optical fiber from a third fiber temperature to a fourthfiber temperature at a second cooling rate, said third fiber temperaturebeing less than or equal to said second fiber temperature, said secondcooling rate being greater than said first cooling rate and less than5000° C./s.

The present disclosure extends to:

A method of processing an optical fiber comprising:

cooling an optical fiber from a first fiber temperature to a secondfiber temperature in a slow cooling device along a process pathway; saidslow cooling device having an entrance, an exit, and a controlledcooling region; said optical fiber entering said slow cooling device atsaid first fiber temperature at said entrance, said optical fiberexiting said slow cooling device at said second fiber temperature atsaid exit; said controlled cooling region including two or more zonesfor processing said optical fiber; each of said zones having an averageambient temperature, a maximum ambient temperature and a minimum ambienttemperature; said average ambient temperature differing in each of saidzones; a difference between said maximum ambient temperature and saidminimum ambient temperature in each of said zones being less than 25°C.; each of said zones cooling said optical fiber at an average coolingrate less than 5000° C./s.

The present disclosure extends to:

A method of processing an optical fiber comprising:

drawing an optical fiber from a preform at a draw speed greater than 42m/s, said optical fiber having a fictive temperature less than 1540° C.

The present disclose extends to:

An apparatus for processing an optical fiber comprising:

a slow cooling device, said slow cooling device having an entrance forreceiving an optical fiber, an exit for delivering an optical fiber, anda controlled cooling region between said entrance and said exit, saidcontrolled cooling including two or more zones for processing an opticalfiber, said two or more zones including a first zone maintained at afirst ambient temperature and a second zone maintained at a secondambient temperature, said first zone being upstream of said second zone,said second ambient temperature being at least 500° C., said firstambient temperature being greater than said second ambient temperatureby at least 100° C.

Additional features and advantages will be set forth in the detaileddescription which follows, and in part will be readily apparent to thoseskilled in the art from the description or recognized by practicing theembodiments as described in the written description and claims hereof,as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are merely exemplary, and areintended to provide an overview or framework to understand the natureand character of the claims.

The accompanying drawings are included to provide a furtherunderstanding, and are incorporated in and constitute a part of thisspecification. The drawings are illustrative of selected aspects of thepresent description, and together with the specification serve toexplain principles and operation of methods, products, and compositionsembraced by the present description. Features shown in the drawing areillustrative of selected embodiments of the present description and arenot necessarily depicted in proper scale.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims particularly pointing outand distinctly claiming the subject matter of the written description,it is believed that the specification will be better understood from thefollowing written description when taken in conjunction with theaccompanying drawings, wherein:

FIG. 1 depicts a fiber processing system and method having a drawfurnace and a slow cooling device.

FIG. 2 depicts a fiber processing system and method having a drawfurnace, a slow cooling device with multiple processing zones, and afiber-turn device.

FIG. 3 depicts the ambient temperature profile for three configurationsof the system and method of FIG. 2.

FIG. 4 depicts modeled fiber temperature and fiber fictive temperaturefor the three configurations having the ambient temperature profilesshown in FIG. 3.

FIG. 5 depicts modeled fiber temperature cooling rates for a slowcooling device with a constant ambient temperature profile and a gradedambient temperature profile.

The embodiments set forth in the drawings are illustrative in nature andnot intended to be limiting of the scope of the detailed description orclaims. Whenever possible, the same reference numeral will be usedthroughout the drawings to refer to the same or like feature.

DETAILED DESCRIPTION

The present disclosure is provided as an enabling teaching and can beunderstood more readily by reference to the following description,drawings, examples, and claims. To this end, those skilled in therelevant art will recognize and appreciate that many changes can be madeto the various aspects of the embodiments described herein, while stillobtaining the beneficial results. It will also be apparent that some ofthe desired benefits of the present embodiments can be obtained byselecting some of the features without utilizing other features.Accordingly, those who work in the art will recognize that manymodifications and adaptations are possible and can even be desirable incertain circumstances and are a part of the present disclosure.Therefore, it is to be understood that this disclosure is not limited tothe specific compositions, articles, devices, and methods disclosedunless otherwise specified. It is also to be understood that theterminology used herein is for the purpose of describing particularaspects only and is not intended to be limiting.

In this specification and in the claims which follow, reference will bemade to a number of terms which shall be defined to have the followingmeanings:

Fiber temperature refers to the average temperature of the core andcladding regions of a glass optical fiber.

Fictive temperature refers to the average fictive temperature of thecore and cladding regions of a glass optical fiber.

Ambient refers to an environment to which an optical fiber is exposedduring processing.

Ambient temperature refers to a temperature of the environment to whichan optical fiber is exposed during processing.

Reference will now be made in detail to illustrative embodiments of thepresent description.

The present description provides optical fibers having low attenuationand low fictive temperature and methods for making such optical fibersthat are implemented at high draw speed. The optical fibers exhibitattenuation of optical signals less than 0.200 dB/km at a wavelength of1550 nm. The optical fibers are prepared by a process that includescontrolled cooling of the optical fiber. The controlled cooling permitsproduction of optical fibers with fictive temperature less than 1550° C.The attenuation and fictive temperature of the optical fibers areachieved in fiber draw processes operated at draw speeds greater than 40m/s.

In various embodiments, the attenuation of the optical fiber at awavelength of 1550 nm is less than 0.200 dB/km, or less than 0.195dB/km, or less than 0.190 dB/km, or less than 0.185 dB/km, or in therange from 0.170 db/km-0.200 dB/km, or in the range from 0.175db/km-0.195 dB/km, or in the range from 0.180 db/km-0.190 dB/km.

In various embodiments, the attenuation of the optical fiber at awavelength of 1310 nm is less than 0.340 dB/km, or less than 0.335dB/km, or less than 0.330 dB/km, or less than 0.325 dB/km, or in therange from 0.310 db/km-0.340 dB/km, or in the range from 0.315db/km-0.335 dB/km, or in the range from 0.320 db/km-0.330 dB/km.

In various embodiments, the fictive temperature of the optical fiber isless than 1550° C., or less than 1545° C., or less than 1540° C., orless than 1535° C., or less than 1530° C., or less than 1525° C., or inthe range from 1500° C.-1550° C., or in the range from 1510° C.-1540°C., or in the range from 1515° C.-1535° C.

Embodiments further extend to optical fibers exhibiting two or more ofthe performance attributes (attenuation of at 1550 nm, attenuation at1310 nm, and fictive temperature) disclosed herein.

Optical fibers in accordance with the embodiments disclosed herein areproduced in fiber draw processes operated at draw speeds greater than 30m/s, or greater than 40 m/s, or greater than 42 m/s, or greater than 45m/s, or greater than 47 m/s, or greater than 50 m/s, or greater than 55m/s, or greater than 60 m/s, or in the range from 30 m/s-70 m/s, or inthe range from 40 m/s-65 m/s, or in the range from 42 m/s-63 m/s, or inthe range from 44 m/s-60 m/s, or in the range from 46 m/s-58 m/s.

In a fiber draw process, optical fibers are formed by drawing a fiberfrom a glass preform. The glass preform is heated to a softened stateand an optical fiber is drawn from the softened preform through theaction of gravity and applied tension. As the fiber is drawn from thepreform, it cools and as it cools, the structure of the glass evolvesfrom a relatively disordered state in the preform to a more orderedstate in the cooled fiber. The driving force for ordering of glassstructure during cooling is a lowering of energy and approach to athermodynamic equilibrium state. The thermodynamic equilibrium statecorresponds to the minimum energy state of the glass. As a glass cools,however, its viscosity increases and the structural rearrangementsneeded for structural relaxation are inhibited. As a result, the timescale needed to reach the equilibrium state is increased.

The extent to which the structure of the glass relaxes over a given timeperiod during cooling depends on the rate of cooling. At fast coolingrates, the viscosity of the glass increases rapidly and the window oftime in which the viscosity is sufficiently low to permit structuralrearrangements is short. As a result, the extent of relaxation of glassstructure is limited, the approach to equilibrium is kineticallyinhibited, and the glass remains in a non-equilibrium state. As thecooling rate decreases, the time window in which is the glass exhibits aviscosity in the range conducive to structural relaxation is increasedand a closer approach of the glass to the equilibrium structural stateis achieved during cooling.

As is known in the art, fictive temperature is an indicator of structurein glasses. As the structure of a glass relaxes and approaches anequilibrium state, the fictive temperature of the glass decreases. For agiven composition, glasses with a high fictive temperature are furtherremoved from equilibrium and have less relaxed structures than glasseswith a low fictive temperature. Relaxation of glass structure isaccompanied by a reduction in fictive temperature.

Processing conditions that lower the fictive temperature of opticalfibers are desirable because optical fibers with low fictivetemperatures exhibit low attenuation. The fictive temperature of anoptical fiber is influenced by controlling the cooling rate of the fiberduring manufacture. Processing stages used to control the cooling rateof an optical fiber are referred to herein as slow cooling devices. Aslow cooling device includes a controlled cooling region thatestablishes an ambient temperature to which the optical fiber is exposedduring cooling. During the draw process, an optical fiber passes throughthe controlled cooling region and the temperature of the fiber isinfluenced by the thermal conditions (including ambient temperature)maintained in the controlled cooling region.

In one embodiment, a heated gas is supplied to the controlled coolingregion and the optical fiber is exposed to the heated gas as it passesthrough the controlled cooling region during processing. The gasenvironment surrounding the optical fiber is referred to herein as theambient or ambient environment. The temperature of the heated gascorresponds to the ambient temperature, where the ambient temperature isless than the temperature of the fiber, but greater than roomtemperature. As a result, the rate of cooling of the fiber in thepresence of the heated gas is slower than the rate of cooling of thefiber in the presence of room temperature air.

In principle, it is possible to achieve any degree of structuralrelaxation and any fictive temperature desired for an optical fiber bycooling the fiber at a sufficiently slow rate. The rate of cooling in aslow cooling device, for example, can be influenced by controlling thedifference between the fiber temperature and the ambient temperature ofthe controlled cooling region. The closer the ambient temperature is tothe fiber temperature, the slower is the cooling rate.

The time of exposure of the fiber to the controlled cooling region,referred to herein as “residence time”, is also important to controllingthe fictive temperature of the fiber. Relaxation of the structure of aglass occurs on a time scale characteristic of the atomic rearrangementsthat occur as the glass approaches an equilibrium state. For a givenglass composition, the time scale for structural relaxation varies withthe viscosity of the glass. When the glass has low viscosity, atomicrearrangements are more facile and the characteristic time scale ofglass relaxation is shorter. In order for the structure of the glass torelax effectively, the viscosity of the glass must be sufficiently lowfor a sufficiently long period of time to enable structuralrearrangement. Since glass viscosity varies with glass temperature,reductions in fictive temperature require processes in which theresidence time of the fiber in a controlled cooling region maintained ata particular ambient temperature is sufficiently long to permitstructural relaxation. To achieve the greatest reduction in fictivetemperature at the particular ambient temperature of the controlledcooling region, the residence time of the fiber in the controlledcooling region should be sufficiently long to relax the structure of theglass to the extent possible at the particular ambient temperature ofthe controlled cooling region. Further reductions in fictive temperaturecan be realized by lowering the fiber temperature by systematicallylowering the ambient temperature of the controlled cooling region (orpassing the fiber through a series of controlled cooling regions withprogressively decreasing ambient temperatures) and insuring that theresidence time of the fiber in the controlled cooling region(s) issufficiently long at each ambient temperature to permit structuralrelaxation to the greatest extent possible.

As the fiber temperature decreases, however, the time scale needed toeffect structural relaxation to a degree capable of meaningfullyreducing fictive temperature increases. At some point during cooling, afiber temperature is reached at which the necessary time scale becomesunreasonably long for manufacturing. At this point, cooling rate is nolonger a practical consideration and it is desirable to decrease thefiber temperature to room temperature at high cooling rates to improveprocess speed.

Preferred fiber temperatures that are conducive to structural relaxationin silica and doped silica are fiber temperatures in the range from1000° C.-1700° C., or fiber temperatures in the range from 1100°C.-1650° C., or fiber temperatures in the range from 1200° C.-1600° C.,or fiber temperatures in the range from 1250° C.-1550° C. or fibertemperatures in the range from 1300° C.-1500° C.

Continuous manufacturing processes, in which a fiber is drawncontinuously at a particular draw speed, are preferred for reasons ofcost and efficiency. Continuous manufacturing processes, however,present additional challenges for lowering fictive temperature becausethe optical fiber is in constant motion. Due to the constant motion, thefiber has a residence time in the controlled cooling region dictated bythe draw speed. Increased residence time can be achieved by drawingfibers at lower speeds, but low draw speeds are not desirable from theperspective of process efficiency. At a particular draw speed, thelength of the controlled cooling region also influences residence time.Due to space constraints in practical manufacturing facilities, however,the length of the controlled cooling region is necessarily limited.

Space constraints introduce further complications because systems usedto process optical fibers include process units in addition to slowcooling devices. In a typical fiber manufacture process, the fiber isdrawn from a preform situated in a draw furnace and routed through aprocess pathway that includes thermal treatment devices (e.g. slowcooling devices), coating devices (e.g. devices that apply liquidcompositions for forming primary coatings, secondary coatings, and inklayers), metrology units (e.g. fiber diameter control), and variousfiber handling devices (e.g. turning devices, take up spools). In orderto reduce the space needed to manufacture optical fibers, it isdesirable to position the process units as close as possible to eachother. Close positioning of process units is complicated by the factthat operation of the different process units requires the fibertemperature to be within a particular range. In order to coat a fiber,for example, the fiber temperature needs to be sufficiently low toprevent vaporization of volatile components of liquid coatingformulations applied to the surface of the fiber. Fiber handling devicesrequire fiber temperatures that are sufficiently low to preserve themechanical integrity of the fiber. If the fiber temperature is too high,bending or winding of a fiber can introduce permanent structuraldeformations that damage or comprise the integrity of the fiber. As aresult, considerations related to fiber cooling extend beyond thetemperature window associated with control of fictive temperature toinclude the lower temperatures needed for coating and handling of thefiber.

The competing demands on the evolution of fiber temperature during thefiber draw process necessitate compromises in the cooling rate of thefiber. On the one hand, slow cooling rates are preferred to further theobjective of lowering the fictive temperature of the fiber. On the otherhand, fast cooling rates are desired to further the objectives ofcompactness of the arrangement of processing units in manufacturing andhigh speed processing.

In conventional fiber draw processes, the competing demands on fibertemperature are resolved by incorporating a slow cooling device with acontrolled cooling region held at a constant ambient temperature in themanufacturing process and adjusting the constant ambient temperature toa level that provides cooling at a sufficient rate to insure that thefiber temperature at the exit of a slow cooling device is sufficientlylow to meet the requirements of a downstream processing unit.

While the compromise in cooling rates embodied in conventional fiberdraw processes leads to a reduction in fiber fictive temperature andprovides optical fibers with satisfactory attenuation for manyapplications, the present disclosure recognizes that further reductionsin fiber fictive temperature and attenuation are possible inspace-constrained high speed fiber drawing processes. In particular, thepresent disclosure demonstrates that lower fiber fictive temperature andlower fiber attenuation can be obtained in processes that utilize a slowcooling device operated at conditions of non-constant ambienttemperature within the controlled cooling region. In particular, thepresent disclosure demonstrates that when the controlled cooling regionof a slow cooling device is modified to include a non-uniform ambienttemperature distribution, reductions in fiber fictive temperature andattenuation are realized relative to a corresponding process operatedwith a slow cooling device having a controlled cooling region held at aconstant ambient temperature.

FIG. 1 illustrates a simplified system for drawing optical fibers thatincludes a draw furnace and a slow cooling device. As defined herein, aslow cooling device provides cooling of an optical fiber at a rate thatis slower than the cooling rate of the optical fiber in stationary airat room temperature.

In one or more embodiments, the ambient temperature of the controlledcooling region or zones within the controlled cooling region can beestablished, varied, or maintained by controlling the temperature,pressure, and/or flow rate of a gas supplied to the ambient environmentsurrounding the fiber.

In one embodiment, the temperature and cooling rate of an optical fiberis controlled by supplying a gas to the controlled cooling region. Thecontrolled cooling region is filled with a static or flowing gas and theambient temperature and fiber cooling rate is varied by controlling thetemperature, pressure and/or flow rate of the gas. Representative gasesfor use in the controlled cooling region of a slow cooling deviceinclude He, Ar, N₂, air, CO₂, Kr, and Xe. To facilitate a decrease inthe fictive temperature of an optical fiber, slower cooling rates arepreferred.

Higher gas pressures and/or faster gas flow rates lead to faster coolingrates, while lower gas pressures and/or slower gas flow rates lead toslower cooling rates. In one embodiment, the gas pressure in the ambientof controlled cooling is less than room pressure. In embodiments, thegas pressure in the controlled cooling region is less than 1.0 atm, orless than 0.9 atm, or less than 0.8 atm, or less than 0.7 atm, or lessthan 0.6 atm, or less than 0.5 atm, or less than 0.4 atm, or less than0.3 atm, or in the range from 0.1 atm-1.0 atm, or in the range from 0.2atm-0.9 atm, or in the range from 0.3 atm-0.8 atm, or in the range from0.4 atm-0.7 atm.

The thermal conductivity of the gas in the ambient surrounding the fiberalso influences the cooling rate. Gases with higher thermal conductivitylead to faster cooling rates, while gases with lower thermalconductivity lead to slower cooling rates. To promote slower coolingrates, the thermal conductivity of the gas at the ambient temperature ofthe ambient surrounding the fiber is less than 2.0×10⁻⁴ cal/cm-s-K, orless than 1.8×10⁻⁴ cal/cm-s-K, or less than 1.6×10⁻⁴ cal/cm-s-K, or lessthan 1.5×10⁻⁴ cal/cm-s-K, or less than 1.4×10⁻⁴ cal/cm-s-K, or in therange 0.5×10⁻⁴ cal/cm-s-K-2.0×10⁻⁴ cal/cm-s-K, or in the range 0.75×10⁻⁴cal/cm-s-K-1.75×10⁻⁴ cal/cm-s-K, or in the range 1.0×10⁻⁴cal/cm-s-K-1.6×10⁻⁴ cal/cm-s-K. In other embodiments, the thermalconductivity of the gas in the ambient surrounding the fiber is lessthan 2.0×10⁻⁴ cal/cm-s-K, or less than 1.8×10⁻⁴ cal/cm-s-K, or less than1.6×10⁻⁴ cal/cm-s-K, or less than 1.5×10⁻⁴ cal/cm-s-K, or less than1.4×10⁻⁴ cal/cm-s-K, or in the range 0.5×10⁻⁴ cal/cm-s-K-2.0×10⁻⁴cal/cm-s-K, or in the range 0.75×10⁻⁴ cal/cm-s-K-1.75×10⁻⁴ cal/cm-s-K,or in the range 1.0×10⁻⁴ cal/cm-s-K-1.6×10⁻⁴ cal/cm-s-K at one or moretemperatures in the range from 800° C.-1500° C. In still otherembodiments, the thermal conductivity of the gas in the ambientsurrounding the fiber is less than 2.0×10⁻⁴ cal/cm-s-K, or less than1.8×10⁻⁴ cal/cm-s-K, or less than 1.6×10⁻⁴ cal/cm-s-K, or less than1.5×10⁻⁴ cal/cm-s-K, or less than 1.4×10⁻⁴ cal/cm-s-K, or in the range0.5×10⁻⁴ cal/cm-s-K-2.0×10⁻⁴ cal/cm-s-K, or in the range 0.75×10⁻⁴cal/cm-s-K-1.75×10⁻⁴ cal/cm-s-K, or in the range 1.0×10⁻⁴cal/cm-s-K-1.6×10⁻⁴ cal/cm-s-K at each temperature in the range from800° C.-1500° C.

The temperature of the gas in the ambient surrounding the optical fiberalso influences the cooling rate. In particular, the difference betweenthe fiber temperature and the ambient temperature established by the gasrepresents a driving force for transfer of heat from the fiber to thegas and influences the rate of cooling. Larger differences between fibertemperature and ambient temperature lead to faster heat transfer andfaster cooling rates, while smaller differences between fibertemperature and ambient temperature lead to slower heat transfer andslower cooling rates. As noted herein, to reduce fiber fictivetemperature it is preferable to process the fiber at a slow rate ofcooling.

To promote slower cooling rates, in one or more embodiments herein, thedifference between the fiber temperature and ambient temperature is lessthan 500° C., or less than 400° C., or less than 300° C., or less than200° C., or in the range from 50° C.-500° C., or in the range from 100°C.-450° C., or in the range from 150° C.-400° C., or in the range from200° C.-300° C., where the fiber temperature is greater than the ambienttemperature. To promote faster cooling rates, in one or more embodimentsherein, the difference between the fiber temperature and ambienttemperature is greater than 200° C., or greater than 300° C., or greaterthan 400° C., or greater than 500° C., or in the range from 200° C.-800°C., or in the range from 300° C.-700° C., or in the range from 400°C.-600° C., where the fiber temperature is greater than the ambienttemperature. In the foregoing embodiments, the fiber temperaturecorresponds to the fiber temperature at any point during cooling or anyposition within the controlled cooling region or zone of a controlledcooling region. The difference in fiber temperature and ambienttemperature disclosed herein apply, for example, to the fibertemperature at the entrance, interior position, or exit of a slowcooling device, or to the fiber temperature at the entrance, interiorposition, or exit of a controlled cooling region, or to the fibertemperature at the entrance, interior position, or exit of a zone of acontrolled cooling region, or to the fiber temperature at the entrance,interior position, or exit of a combination of two or more zones of acontrolled cooling region.

System 100 includes draw furnace 105 with fiber preform 110. In oneembodiment, fiber preform 110 is a silica or doped silica preform thatincludes a single compositional region or multiple compositionalregions. The multiple compositional regions may be concentric. Forexample, fiber preform 110 may include a central region corresponding tothe core composition of a fiber and one or more outer concentric regionscorresponding to the compositions of one or more cladding layers of afiber. The core and/or cladding regions may include pure silica or dopedsilica.

Optical fiber 115 is drawn from fiber preform 110 and directed toentrance 123 of slow cooling device 120. Optical fiber 115 passesthrough slow cooling device 120 and emerges at exit 127. The temperatureof optical fiber 115 at exit 127 is less than the temperature of opticalfiber 115 at entrance 123. Slow cooling device 120 includes an internalcontrolled cooling region that establishes an ambient temperature towhich the fiber is exposed. Slow cooling device 120 effects controlledcooling of fiber 315 and enables cooling rates that are slower than thenatural cooling rate of optical fiber 115 in stationary unheated air. Inone embodiment, optical fiber 115 exits draw furnace 105 at a fibertemperature of 1700° C. Optical fiber 115 exits draw furnace 105 andproceeds to slow cooling device 120. Optical fiber 115 continues to coolas it proceeds along the process pathway after emerging from slowcooling device 120 at exit 127.

System 100 defines a process pathway along which the fiber is directed.The process pathway is the route traversed by the fiber in a fiber drawprocess. The process pathway of system 100 extends from draw furnace 105to beyond exit 127 of slow cooling device 120. The arrow defines thedirection of conveyance of optical fiber 115 along the process pathway.As optical fiber 115 is processed, it exits draw furnace 105 andproceeds along the process pathway. Positions along the process pathwaythat are closer to the point of exit of the fiber from draw furnace 105are said herein to be upstream of positions along the process pathwaythat are further away from the point of exit of the fiber from drawfurnace 105, where distance from draw furnace 105 is understood hereinto mean distance as measured along optical fiber 115. The direction ofconveyance of the optical fiber is the downstream direction; the opticalfiber is conveyed from upstream positions to downstream positions alongthe process pathway. For example, the portion of optical fiber 115positioned between draw furnace 105 and entrance 123 of slow coolingdevice 120 is upstream of the portion of optical fiber 115 positionedfurther from draw furnace 105 than exit 127 of slow cooling device 120.Similarly, entrance 123 of slow cooling device 120 is upstream of exit127 of slow cooling device 120 and draw furnace 105 is upstream of slowcooling device 120. Since optical fiber 115 passes through both drawfurnace 105 and slow cooling device 120 along the process pathway, drawfurnace 105 and slow cooling device 120 may be referred to herein asbeing operatively coupled along the process pathway.

Although not explicitly shown in FIG. 1, the fiber process pathway mayfurther include other processing units positioned upstream or downstreamfrom slow cooling device 120 (e.g. reheating stages, additional slowcooling devices, metrology units, fiber-turning devices, coating units,testing units, spooling units etc.) along the process pathway.

The draw speed (speed of conveyance) of an optical fiber along throughslow cooling devices disclosed herein is greater than 30 m/s, or greaterthan 40 m/s, or greater than 42 m/s, or greater than 45 m/s, or greaterthan 47 m/s, or greater than 50 m/s, or greater than 55 m/s, or greaterthan 60 m/s, or in the range from 30 m/s-70 m/s, or in the range from 40m/s-65 m/s, or in the range from 42 m/s-63 m/s, or in the range from 44m/s-60 m/s, or in the range from 46 m/s-58 m/s.

Slow cooling device 120 includes a controlled cooling region designed todecrease the cooling rate of optical fiber 115 relative to the coolingrate in unheated air. The controlled cooling region is internal to slowcooling device 120 and encompasses the entirety of the internal volumeof slow cooling device 120 or a portion thereof. Optical fiber 115passes through the controlled cooling region as it proceeds along theprocess pathway from entrance 123 of slow cooling device 120 to exit 127of slow cooling device 120. In various embodiments, the ambienttemperature of the controlled cooling region is established with heatingelements and/or heated gas flow in the environment near optical fiber115.

In the conventional design, the controlled cooling region is maintainedat a constant ambient temperature. The constant ambient temperature is atemperature between room temperature and the temperature of opticalfiber 115 at entrance 123. The constant ambient temperature ispreferably a temperature that permits (i) structural relaxation ofoptical fiber 115, (ii) a lowering of the fictive temperature of opticalfiber 115, and/or (iii) closer approach of optical fiber 115 to anequilibrium state during cooling as described hereinabove.

A slow cooling device in accordance with the present descriptionincludes a controlled cooling region with a non-constant ambienttemperature. Instead of a constant or uniform distribution of ambienttemperature in the controlled cooling region, the controlled coolingregion includes a gradient in ambient temperature. In one embodiment,the ambient temperature profile is non-monotonic and includes one ormore step changes. A graded ambient temperature profile is motivated byrecognition that the time scale for structural relaxation is shorter forhigher fiber temperatures and progressively increases as the fibertemperature decreases. The graded ambient temperature profile isaccordingly designed to provide higher ambient temperatures in theportion of the controlled cooling region closer to the entrance of theslow cooling device and lower ambient temperatures in the portion of thecontrolled cooling device closer to the exit of the slow cooling device.

The fiber temperature at the entrance of the slow cooling device ishigher than the fiber temperature at the exit of the slow coolingdevice. The fiber temperature at the entrance of the slow cooling deviceis greater than 1400° C., or greater than 1500° C., or greater than1600° C., or greater than 1700° C., or in the range from 1400° C.-1900°C., or in the range from 1450° C.-1800° C., or in the range from 1500°C.-1700° C. The fiber temperature at the exit of the slow cooling deviceis less than 1350° C., or less than 1300° C., or less than 1250° C., orless than 1200° C., or in the range from 1000° C.-1350° C., or in therange from 1050° C.-1300° C., or in the range from 1100° C.-1250° C., orin the range from 1000° C.-1250° C.

The higher fiber temperature at the entrance of the slow cooling devicecorresponds to a lower viscosity and more facile structuralrearrangements upon cooling for the optical fiber at the entrance of theslow cooling device relative to the exit of the slow cooling device.Accordingly, structural relaxation is enhanced by maintaining a slowercooling rate in portions of the controlled cooling region closer to theentrance of the slow cooling device and to increase the cooling rate asneeded in portions of the controlled cooling region closer to the exitof the slow cooling device to achieve a desired exit fiber temperature.The increase in cooling rate as the fiber approaches the exit of theslow cooling device can be designed to provide a fiber having a targetedfiber temperature at the exit of the slow cooling device. As notedabove, the fiber temperature at the exit of the slow cooling device canbe adjusted for compatibility with entrance conditions for downstreamprocessing units in the fiber draw process.

Within the slow cooling device or controlled cooling region, portions incloser proximity to the entrance are said to be upstream of portions incloser proximity of the exit. As described more fully below, in someembodiments, the slow cooling device or controlled cooling regionincludes one or more zones for processing an optical fiber. The zonesare discrete, separate processing regions located within the slowcooling device or controlled cooling region along the process pathway.An optical fiber enters the slow cooling device or controlled coolingregion at an entrance and is conveyed downstream through the one or morezones to the exit of the slow cooling device or controlled coolingregion. When two or more zones are present, the zones differ in positionrelative to the entrance and exit of the slow cooling device orcontrolled cooling region. The position of a zone relative to anotherzone is specified as upstream or downstream along the process pathway. Aparticular zone is upstream of a different zone if the particular zoneis positioned in closer proximity to the entrance of the slow coolingdevice or controlled cooling region along the process pathway than thedifferent zone. Similarly, a particular zone is downstream of adifferent zone if the particular zone is positioned in closer proximityto the exit of the slow cooling device or controlled cooling regionalong the process pathway than the different zone.

In the graded ambient temperature profile of the present slow coolingdevice, the ambient temperature distribution is skewed to provide higherambient temperatures near the entrance of the slow cooling device and inupstream portions of the controlled cooling region to promote longerresidence times for the fiber at higher fiber temperatures. By skewingthe ambient temperature profile in this way, the lower viscosity stateof the fiber is maintained for a longer time, greater structuralrelaxation occurs, and lower fiber fictive temperatures are achieved. Toachieve sufficient cooling to insure compatibility of the fiber exitingthe slow cooling device with downstream processing units, the ambienttemperature distribution is skewed to ambient temperatures in thedownstream portion of the controlled cooling region sufficient tocompensate for the slower cooling rates associated with the higherambient temperatures maintained in the upstream portion of thecontrolled cooling region.

Several observations of the present slow cooling device relative toconventional slow cooling devices are noteworthy. In the conventionalslow cooling device, the controlled cooling region is maintained at aconstant ambient temperature. The constant ambient temperature isselected to insure that the fiber temperature at the exit of the slowcooling device is compatible for further processing by downstream units.Factors that influence selection of the constant ambient temperatureinclude the fiber temperature at the entrance of the slow coolingdevice, the length of the slow cooling device, draw speed, and manner ofcontrol of the ambient temperature (e.g. thermal conductivity, flowrate, and/or pressure of the gas supplied to the controlled coolingregion).

The constant ambient temperature selected for the conventional slowcooling device reflects a compromise of several competingconsiderations. The present description recognizes that in order toproperly balance the competing considerations, the constant ambienttemperature selected for the controlled cooling region of a conventionalslow cooling device is necessarily less than optimal for promoting themore efficient structural relaxation that occurs at the higher fibertemperatures present in the portion of the controlled cooling regionnear the entrance of the slow cooling device. Relative to the fibertemperature at the entrance of the slow cooling device, the constantambient temperature is sufficiently low to cool the fiber at a rate thatshortens the residence time of the fiber at the higher fibertemperatures that are more conducive to structural relaxation.

In the present slow cooling device, the ambient temperature profile ofthe controlled cooling region includes higher ambient temperatures inupstream portions of the controlled cooling region and lower ambienttemperatures in downstream portions of the controlled cooling region.The higher ambient temperatures in upstream portions slow the coolingrate of the fiber and extend the time at which the fiber is at a fibertemperature sufficient to maintain a viscosity suitable for efficientstructural relaxation. To achieve a desired fiber temperature at theexit of the slow cooling device, the ambient temperature of downstreamportions of the controlled cooling region of the present slow coolingdevice is lowered sufficiently to increase the cooling rate to thedegree necessary to provide a targeted exit fiber temperature.

For slow cooling devices of a given design and length having the sameentrance fiber temperature and same exit fiber temperature in a processof a given draw speed, the ambient temperatures in the upstream portionsof the controlled cooling region of the present slow cooling device arehigher than the constant ambient temperature that would be utilized fora corresponding conventional slow cooling device. The higher upstreamambient temperatures slow the cooling rate of the fiber in the portionsof the slow cooling device at which the fiber temperature is higher, sogreater structural relaxation occurs in the upstream portions of thecontrolled cooling region of the present slow cooling device relative toa corresponding conventional slow cooling device.

It is recognized, however, that in order to achieve a particular fiberexit temperature, the ambient temperatures in the downstream portions ofthe controlled cooling region of the present slow cooling device need tobe lower than the constant ambient temperature required to achieve thesame fiber exit temperature for a corresponding conventional slowcooling device. Although the lower ambient temperatures lead to fastercooling rates of the fiber in the downstream portions of the controlledcooling region of the present slow cooling device and less efficientstructural relaxation relative to a corresponding conventional slowcooling device, the less efficient structural relaxation occurs in afiber temperature regime in which the time scale for structuralrelaxation of the fiber is too long to allow for a meaningful reductionin fictive temperature. As a result, even though less structuralrelaxation occurs in the downstream portions of the controlled coolingregion of the present slow cooling device relative to a correspondingconventional prior art slow cooling device, the difference in structuralrelaxation is immaterial and more than compensated by the largerdifference in structural relaxation that occurs in the upstream portionsof the controlled cooling region of the present slow cooling devicerelative to the corresponding conventional slow cooling device, wherethe fiber temperature is such that the fiber viscosity is low andstructural relaxation is efficient. Lower fiber fictive temperatureaccordingly results from fibers processed by the present slow coolingdevice relative to a corresponding conventional slow cooling device.

The ambient temperature profile of the controlled cooling region of thepresent slow cooling device is designed to promote structural relaxationfor longer times when the fiber temperature is high relative to acorresponding conventional slow cooling device. The longer times areachieved by cooling the fiber at slower rates in the upstream portionsof the controlled cooling region of the present slow cooling devicerelative to a corresponding conventional slow cooling device. Thetradeoff is a faster cooling rate and less structural relaxation in thedownstream portions of the controlled cooling region of the present slowcooling device relative to a corresponding conventional slow coolingdevice. On balance, however, a greater net reduction in fiber fictivetemperature occurs in the present slow cooling device relative to acorresponding conventional slow cooling device due to a skewing ofambient temperatures in the controlled cooling region away from constantambient temperature to a distribution that provides slower cooling rateswhen the fiber has a higher fiber temperature and lower viscosity, andfaster cooling rates when the fiber has a lower fiber temperature andhigher viscosity. The ambient temperature distribution of the presentslow cooling device provides fibers with lower fictive temperature whilestill enabling adjustment of fiber exit temperature as needed to insurecompatibility with downstream processing units.

In one embodiment, the ambient temperature gradient of the controlledcooling region of the present slow cooling device is continuous withcontinuous variations in ambient temperature in the direction of fiberconveyance through the controlled cooling region. The continuousvariation in ambient temperature has a constant or variable slope acrossthe controlled cooling region in the direction of fiber conveyance.

In another embodiment, the ambient temperature gradient of thecontrolled cooling region of the present slow cooling device includesstep changes or discontinuities in ambient temperature. In oneembodiment, the controlled cooling region includes two or more zones,where the ambient temperature within a zone is constant or approximatelyconstant and the constant ambient temperatures in different zonesdiffer. The number of zones is at least two, or at least three, or atleast four, or at least five, or at least six, or in the range from2-10, or in the range from 3-9, or in the range from 4-8.

In another embodiment, the controlled cooling region includes two ormore zones, where the ambient temperature varies within each zone andthe average ambient temperatures in different zones differ. Thevariation in ambient temperature in each zone extends from a maximumambient temperature to a minimum ambient temperature within the spatialdimensions of the zone, where the average ambient temperature isselected as the average of the maximum ambient temperature and theminimum ambient temperature. The difference between the maximum ambienttemperature and minimum ambient temperature within a zone is less than30° C., or less than 25° C., or less than 20° C., or in the range from0° C.-30° C., or in the range from 5° C.-30° C., or in the range from 5°C.-25° C., or in the range from 5° C.-20° C., or in the range from 10°C.-30° C., or in the range from 10° C.-25° C. The number of zones is atleast two, or at least three, or at least four, or at least five, or atleast six, or in the range from 2-10, or in the range from 3-9, or inthe range from 4-8. Further embodiments include controlled coolingregions with two or more zones that include one or more zones having aconstant ambient temperature and one or more zones that include avariation in ambient temperature.

In one or more embodiments, the two or more zones include an upstreamzone and a downstream zone, where the constant ambient temperature inthe upstream zone is greater than the constant ambient temperature inthe downstream zone by at least 100° C., or at least 150° C., or atleast 200° C., or at least 250° C., or at least 300° C., or at least350° C., or at least 400° C., or by an amount in the range from 100°C.-500° C., or by an amount in the range from 150° C.-450° C., or by anamount in the range from 200° C.-400° C., or by an amount in the rangefrom 125° C.-300° C., or by an amount in the range from 150° C.-250° C.In one embodiment, the upstream zone and downstream zone are adjacentwith no intervening zones. In another embodiment, the upstream zone anddownstream zone are separated by one or more intervening zones. In stillanother embodiment, the upstream zone is adjacent to the entrance of thefiber to the slow cooling device. In yet another embodiment, thedownstream zone is adjacent to the exit of the fiber from the slowcooling device.

In one or more embodiments, the two or more zones include an upstreamzone and a downstream zone, where the average ambient temperature in theupstream zone is greater than the average ambient temperature in thedownstream zone by at least 100° C., or at least 150° C., or at least200° C., or at least 250° C., or at least 300° C., or at least 350° C.,or at least 400° C., or by an amount in the range from 100° C.-500° C.,or by an amount in the range from 150° C.-450° C., or by an amount inthe range from 200° C.-400° C., or by an amount in the range from 125°C.-300° C., or by an amount in the range from 150° C.-250° C. In oneembodiment, the upstream zone and downstream zone are adjacent with nointervening zones. In another embodiment, the upstream zone anddownstream zone are separated by one or more intervening zones. In stillanother embodiment, the upstream zone is adjacent to the entrance of thefiber to the slow cooling device. In yet another embodiment, thedownstream zone is adjacent to the exit of the fiber from the slowcooling device.

In one or more embodiments, the ambient temperature to which the opticalfiber is exposed during the draw process is a temperature of at least500° C., or at least 600° C., or at least 700° C., or at least 800° C.,or at least 900° C., at least 1000° C., or at least 1100° C., or atemperature in the range from 500° C.-1200° C., or a temperature in therange from 600° C.-1100° C., or a temperature in the range from 700°C.-1000° C. In one embodiment, the ambient temperature is a constanttemperature maintained throughout a zone of the controlled coolingregion or slow cooling device.

The cooling rate of the fiber temperature in the controlled coolingregion of the present slow cooling device is constant or variable,continuous or discontinuous, and/or monotonic or non-monotonic. Thecooling rate is preferably variable with a slower cooling rate inportions of the controlled cooling region proximate to the entry pointof the fiber into the slow cooing device and a faster cooling rate inportions of the controlled cooling region proximate to the exit point ofthe fiber from the slow cooling device. In one embodiment, the coolingrate in upstream portions of the controlled cooling region is slowerthan the cooling rate in downstream portions of the controlled coolingregion.

In one or more embodiments, the controlled cooling region includes twoor more zones and the cooling rate within each zone is constant and theconstant cooling rate differs in different zones. The cooling ratewithin a zone is monotonic or non-monotonic and the transition ofcooling rate from one zone to another zone is continuous ordiscontinuous.

In one or more embodiments, the controlled cooling region includes twoor more zones in which the constant cooling rates of fiber temperaturediffer in each of the two or more zones, where the constant cooling rateof fiber temperature in each of the two or more zones is less than 5000°C./s, or less than 4000° C./s, or less than 3500° C./s, or less than3000° C./s, or less than 2500° C./s, or less than 2000° C./s, or lessthan 1500° C./s, or in the range from 1000° C./s-4500° C./s, or in therange from 1500° C./s-4000° C./s, or in the range from 2000° C./s-3500°C./s.

In one or more embodiments, the controlled cooling region includes anupstream zone and a downstream zone in which the constant cooling ratesof fiber temperature differ, where the constant cooling rate of fibertemperature in each of the upstream zone and downstream zone is lessthan 5000° C./s, or less than 4000° C./s, or less than 3500° C./s, orless than 3000° C./s, or less than 2500° C./s, or less than 2000° C./s,or less than 1500° C./s, or in the range from 1000° C./s-4500° C./s, orin the range from 1500° C./s-4000° C./s, or in the range from 2000°C./s-3500° C./s. In one embodiment, the upstream zone and downstreamzone are adjacent with no intervening zones. In another embodiment, theupstream zone and downstream zone are separated by one or moreintervening zones. In still another embodiment, the upstream zone isadjacent to the entrance of the fiber to the slow cooling device. In yetanother embodiment, the downstream zone is adjacent to the exit of thefiber from the slow cooling device.

In one or more embodiments, the controlled cooling region includes anupstream zone and a downstream zone in which the constant cooling ratesof fiber temperature differ, where the constant cooling rates of fibertemperature in each of the upstream zone and downstream zone is lessthan 5000° C./s, or less than 4000° C./s, or less than 3500° C./s, orless than 3000° C./s, or less than 2500° C./s, or less than 2000° C./s,or less than 1500° C./s, or in the range from 1000° C./s-4500° C./s, orin the range from 1500° C./s-4000° C./s, or in the range from 2000°C./s-3500° C./s, and where the constant cooling rate in the downstreamzone is greater than the constant cooling rate in the upstream zone byat least 250° C./s, or at least 500° C./s, or at least 750° C./s, or atleast 1000° C./s, or by an amount in the range from 250° C./s-2000°C./s, or 500° C./s-1750° C./s, or 750° C./s-1500° C./s. In oneembodiment, the upstream zone and downstream zone are adjacent with nointervening zones. In another embodiment, the upstream zone anddownstream zone are separated by one or more intervening zones. In stillanother embodiment, the upstream zone is adjacent to the entrance of thefiber to the slow cooling device. In yet another embodiment, thedownstream zone is adjacent to the exit of the fiber from the slowcooling device.

In one or more embodiments, the controlled cooling region includes anupstream zone and a downstream zone in which the constant cooling ratesof fiber temperature differ, where the constant cooling rates of fibertemperature in each of the upstream zone and downstream zone is lessthan 5000° C./s, or less than 4000° C./s, or less than 3500° C./s, orless than 3000° C./s, or less than 2500° C./s, or less than 2000° C./s,or less than 1500° C./s, or in the range from 1000° C./s-4500° C./s, orin the range from 1500° C./s-4000° C./s, or in the range from 2000°C./s-3500° C./s, and where the constant cooling rate in the downstreamzone is greater than the constant cooling rate in the upstream zone byat least 5%, or at least 10%, or at least 15%, or at least 20%, or by anamount in the range from 5%-25%, or in the range from 10%-20%. In oneembodiment, the upstream zone and downstream zone are adjacent with nointervening zones. In another embodiment, the upstream zone anddownstream zone are separated by one or more intervening zones. In stillanother embodiment, the upstream zone is adjacent to the entrance of thefiber to the slow cooling device. In yet another embodiment, thedownstream zone is adjacent to the exit of the fiber from the slowcooling device.

In one or more embodiments, the controlled cooling region includes twoor more zones and the cooling rate within each zone is variable and theaverage cooling rate differs in different zones. The variation incooling rate in each zone extends from a maximum cooling rate to aminimum cooling rate within the spatial dimensions of the zone, wherethe average cooling rate is selected as the average of the maximumcooling rate and the minimum cooling rate. The difference between themaximum cooling rate and minimum cooling rate within a zone is less than100° C./s, or less than 75° C./s, or less than 50° C./s, or less than25° C./s, or in the range from 0° C./s-100° C./s, or in the range from10° C./s-90° C./s, in the range from 10° C./s-50° C./s, or in the rangefrom 20° C./s-80° C./s, or in the range from 20° C./s-60° C./s, Thecooling rate within a zone is monotonic or non-monotonic and thetransition of cooling rate from one zone to another zone is continuousor discontinuous.

In one or more embodiments, the controlled cooling region includes twoor more zones in which the average cooling rates of fiber temperaturediffer in each of the two or more zones, where the average cooling rateof fiber temperature in each of the two or more zones is less than 5000°C./s, or less than 4000° C./s, or less than 3500° C./s, or less than3000° C./s, or less than 2500° C./s, or less than 2000° C./s, or lessthan 1500° C./s, or in the range from 1000° C./s-4500° C./s, or in therange from 1500° C./s-4000° C./s, or in the range from 2000° C./s-3500°C./s.

In one or more embodiments, the controlled cooling region includes anupstream zone and a downstream zone in which the average cooling ratesof fiber temperature differ, where the average cooling rate of fibertemperature in each of the upstream zone and downstream zone is lessthan 5000° C./s, or less than 4000° C./s, or less than 3500° C./s, orless than 3000° C./s, or less than 2500° C./s, or less than 2000° C./s,or less than 1500° C./s, or in the range from 1000° C./s-4500° C./s, orin the range from 1500° C./s-4000° C./s, or in the range from 2000°C./s-3500° C./s. In one embodiment, the upstream zone and downstreamzone are adjacent with no intervening zones. In another embodiment, theupstream zone and downstream zone are separated by one or moreintervening zones. In still another embodiment, the upstream zone isadjacent to the entrance of the fiber to the slow cooling device. In yetanother embodiment, the downstream zone is adjacent to the exit of thefiber from the slow cooling device.

In one or more embodiments, the controlled cooling region includes anupstream zone and a downstream zone in which the average cooling ratesof fiber temperature differ, where the average cooling rates of fibertemperature in each of the upstream zone and downstream zone is lessthan 5000° C./s, or less than 4000° C./s, or less than 3500° C./s, orless than 3000° C./s, or less than 2500° C./s, or less than 2000° C./s,or less than 1500° C./s, or in the range from 1000° C./s-4500° C./s, orin the range from 1500° C./s-4000° C./s, or in the range from 2000°C./s-3500° C./s, and where the average cooling rate in the downstreamzone is greater than the average cooling rate in the upstream zone by atleast 250° C./s, or at least 500° C./s, or at least 750° C./s, or atleast 1000° C./s, or by an amount in the range from 250° C./s-2000°C./s, or 500° C./s-1750° C./s, or 750° C./s-1500° C./s. In oneembodiment, the upstream zone and downstream zone are adjacent with nointervening zones. In another embodiment, the upstream zone anddownstream zone are separated by one or more intervening zones. In stillanother embodiment, the upstream zone is adjacent to the entrance of thefiber to the slow cooling device. In yet another embodiment, thedownstream zone is adjacent to the exit of the fiber from the slowcooling device.

In one or more embodiments, the controlled cooling region includes anupstream zone and a downstream zone in which the average cooling ratesof fiber temperature differ, where the average cooling rates of fibertemperature in each of the upstream zone and downstream zone is lessthan 5000° C./s, or less than 4000° C./s, or less than 3500° C./s, orless than 3000° C./s, or less than 2500° C./s, or less than 2000° C./s,or less than 1500° C./s, or in the range from 1000° C./s-4500° C./s, orin the range from 1500° C./s-4000° C./s, or in the range from 2000°C./s-3500° C./s, and where the average cooling rate in the downstreamzone is greater than the average cooling rate in the upstream zone by atleast 5%, or at least 10%, or at least 15%, or at least 20%, or by anamount in the range from 5%-25%, or in the range from 10%-20%. In oneembodiment, the upstream zone and downstream zone are adjacent with nointervening zones. In another embodiment, the upstream zone anddownstream zone are separated by one or more intervening zones. In stillanother embodiment, the upstream zone is adjacent to the entrance of thefiber to the slow cooling device. In yet another embodiment, thedownstream zone is adjacent to the exit of the fiber from the slowcooling device.

In further embodiments, the controlled cooling region of the slowcooling device includes three or more zones, or four or more zones, orfive or more zones, or six or more zones, or between or 2 zones and 10zones, or between 3 zones and 9 zones, or between 4 zones and 8 zones,where any pair or multiple pairs of zones differ in constant ambienttemperature, average ambient temperature, constant cooling rate, and/oraverage cooling rate as disclosed herein. The pair of zones or any pairor pairs of zones within the multiple pairs of zones are adjacent in oneembodiment and non-adjacent in another embodiment. In furtherembodiments, pairs of zones within multiple pairs of zones include anadjacent pair of zones and a non-adjacent pair of zones.

In one or more embodiments, methods of processing an optical fiberinclude cooling an optical fiber from a fiber temperature in the rangefrom 1400° C.-1900° C., or a fiber temperature in the range from 1450°C.-1800° C., or a fiber temperature in the range from 1500° C.-1700° C.to a fiber temperature in the range from 1000° C.-1450° C., or a fibertemperature in the range from 1000° C.-1400° C., or a fiber temperaturein the range from 1000° C.-1300° C., or a fiber temperature in the rangefrom 1000° C.-1250° C., or a fiber temperature in the range from 1000°C.-1200° C., at a cooling rate less than 5000° C./s, or less than 4000°C./s, or less than 3500° C./s, or less than 3000° C./s, or less than2500° C./s, or less than 2000° C./s, or less than 1500° C./s, or in therange from 1000° C./s-4500° C./s, or in the range from 1500° C./s-4000°C./s, or in the range from 2000° C./s-3500° C./s. Each of theseembodiments are performable in one or more zones of a controlled coolingregion of a slow cooling device and are performable in combination withother embodiments disclosed herein in different zones of a controlledcooling region of a slow cooling device, where the different zones areadjacent or separated by one or more intervening zones.

In one or more embodiments, methods of processing an optical fiberinclude cooling an optical fiber from a fiber temperature in the rangefrom 1300° C.-1650° C., or a fiber temperature in the range from 1350°C.-1600° C., or a fiber temperature in the range from 1400° C.-1550° C.to a fiber temperature in the range from 1150° C.-1450° C., or a fibertemperature in the range from 1200° C.-1400° C., or a fiber temperaturein the range from 1250° C.-1350° C. at a cooling rate less than 5000°C./s, or less than 4000° C./s, or less than 3500° C./s, or less than3000° C./s, or less than 2500° C./s, or less than 2000° C./s, or lessthan 1500° C./s, or in the range from 1000° C./s-4500° C./s, or in therange from 1500° C./s-4000° C./s, or in the range from 2000° C./s-3500°C./s. Each of these embodiments are performable in one or more zones ofa controlled cooling region of a slow cooling device and are performablein combination with other embodiments disclosed herein in differentzones of a controlled cooling region of a slow cooling device, where thedifferent zones are adjacent or separated by one or more interveningzones.

In some embodiments, the optical fiber is directed to a downstreamprocessing unit upon exiting a slow cooling device. The fibertemperature at the entrance of a processing unit downstream from theexit of the slow cooling device is less than 1150° C., or less than1125° C., or less than 1100° C., or less than 1075° C., or less than1050° C., or less than 1025° C., or less than 1000° C., or in the rangefrom 950° C.-1150° C., or in the range from 975° C.-1125° C., in therange from 1000° C.-1100° C.

To illustrate selected benefits associated with the present slow coolingdevice, representative examples are considered.

Example 1

In this example, a slow cooling device having a controlled coolingregion with six zones is considered. A schematic of the process is shownin FIG. 2. System 200 includes draw furnace 205 with fiber preform 210.Optical fiber 215 is drawn from fiber preform 210, exits draw furnace205 at exit 212, and is directed to entrance 223 of slow cooling device220. Optical fiber 215 passes through slow cooling device 220, emergesat exit 227, and is directed to fiber-turn device 230. Optical fiber 215enters fiber-turn device 230 at entrance 232 and exits fiber-turn device230 at exit 234 for delivery to optional downstream processing units(not shown). The arrows show the direction of conveyance of opticalfiber 215.

Distance along the process pathway is measured from a furnace base platelocated a short distance upstream from exit 212 of draw furnace 205. Inthe following discussion, the location of the furnace base plate isselected as the zero of position and position increases in thedownstream direction along the process pathway away from the furnacebase plate through exit 212 to slow cooling device 220. Entrance 223 ofslow cooling device 220 is located downstream from exit 212. Slowcooling device 220 has a controlled cooling region that includes 6 zonesnumbered 1-6 in the downstream direction of the process pathway ofoptical fiber 215, as shown in FIG. 2. The length of each zone is 1meter and the process pathway of the fiber extends through the center ofeach zone. The entrance to the controlled cooling region is located ashort distance downstream from entrance 223 of slow cooling device 220.The exit from the controlled cooling region is located a short distanceupstream from exit 227 of slow cooling device 220. Exit 227 is located600 cm downstream of entrance 223. Entrance 232 of fiber-turn device 230is located downstream of exit 227, at a position of 900 cm.

For purposes of illustration, the temperature of optical fiber 215 atentrance 223 to slow cooling device 220 was selected to be 2000° C. andthe temperature of optical fiber 215 at entrance 232 of fiber-turndevice 230 was selected to be about 1075° C. Optical fiber 215 had aGe-doped silica core (average Ge doping concentration of 8 wt %) with acore diameter of approximately 6 microns and a silica cladding with anouter diameter of 125 microns.

The process configurations are considered in this example. In a firstconfiguration (Configuration 1), the fiber draw speed was set to 42 m/sand each zone of the controlled cooling region of slow cooling device220 was set at a constant ambient temperature of 1015° C. In a secondconfiguration (Configuration 2), the fiber draw speed was set to 45 m/sand each zone of the controlled cooling region of slow cooling device220 was set at a constant ambient temperature of 945° C. Configurations1 and 2 are representative of conventional slow cooling devices. Theconstant ambient temperatures were selected to minimize fiber fictivetemperature at each draw speed. In a third configuration (Configuration3), the fiber draw speed was set to 45 m/s and a gradient ambienttemperature profile was used with slow cooling device 220. Each of thesix zones of the controlled cooling region of slow cooling device 220was set to a constant ambient temperature, but the ambient temperatureof some zones differed from the ambient temperature of other zones.Zones 1-3 were set at a constant ambient temperature of 1200° C. Zone 4was set at a constant ambient temperature of 960° C. Zones 5 and 6 wereset at a constant ambient temperature of 600° C. The temperatureprofiles for each of the three configurations are shown in FIG. 3 andthe conditions are summarized in Table 1. In FIG. 3, ambient temperatureprofiles for Configurations 1, 2, and 3 are shown by traces 251, 252,and 253, respectively. Distance is shown as distance from entrance 223of slow cooling device 220. The length of each zone corresponds to adistance of 1 m.

TABLE 1 Configuration 1 Configuration 2 Configuration 3 (42/m/s) (45m/s) (45 m/s) Zone T (° C.) T (° C.) T (° C.) 1 1015 945 1200 2 1015 9451200 3 1015 945 1200 4 1015 945 960 5 1015 945 600 6 1015 945 600

Fiber temperature and fiber fictive temperature were modeled for each ofConfigurations 1-3. The results are shown in FIG. 4. Distance ismeasured from a furnace base plate located a short distance upstreamfrom exit 212 of draw furnace 205. The fibers enter slow cooling device220 at a distance of 119 cm (Configurations 1 and 2) or 153 cm(Configuration 3), exit slow cooling device 220 at a distance of 734 cm(Configurations 1 and 2) or 768 cm (Configuration 3), and enterfiber-turn device 230 at a distance of 900 cm. Solid traces depict fibertemperature and are associated with the temperature scale shown in theleft abscissa. Dashed traces depict fiber fictive temperature and areassociated with the temperature scale shown in the right abscissa.Traces 261, 262, and 263 show fiber temperature for Configurations 1, 2,and 3; respectively. Traces 264, 265, and 266 show fiber fictivetemperature for Configurations 1, 2, and 3; respectively. The fibertemperature at entrance 223 of slow cooling device 220 was 1707° C.,1732° C., and 1624° C. for Configurations 1, 2, and 3; respectively. Thefiber temperature at exit 227 of slow cooling device 220 was 1295° C.,1279° C., and 1230° C. for Configurations 1, 2, and 3; respectively. Thefiber temperature at entrance 232 of fiber-turn device 230 was 1078° C.,1075° C., and 1075° C. for Configurations 1, 2, and 3; respectively. Thefiber fictive temperature was essentially established at exit 227 ofslow cooling device 220 and remained constant along the process pathwaydownstream of slow cooling device 220. The fiber fictive temperature atwas 1534° C., 1542° C., and 1528° C. for Configurations 1, 2, and 3;respectively.

The change in fiber attenuation AAT at wavelength λ relative to areference condition can be calculated from fiber fictive temperatureusing Eq. (1)

$\begin{matrix}{{\Delta AT} = {\frac{A_{\lambda}}{\lambda^{4}}\lbrack {1 - \frac{T_{f}}{T_{f,{ref}}}} \rbrack}} & (1)\end{matrix}$

where A_(λ) is the Rayleigh spectral constant, T_(f) is fiber fictivetemperature, and T_(f,ref) is the fiber fictive temperature at thereference condition. For purposes of comparing fibers produced fromConfigurations 1, 2, and 3, the fiber of Configuration 1 was selected asthe reference fiber and a wavelength of 1310 nm was selected to compareattenuation. At 1310 nm, A_(λ)=A_(1310 nm)=0.907 dB/km-nm⁴. Using thefictive temperatures for fibers produced by Configurations 1, 2, and 3,Eq. (1) yields changes in attenuation of 0.00034 dB/km and −0.00020dB/km for Configurations 2 and 3, respectively, relative toConfiguration 1. Fiber attenuation increases for Configuration 2relative to Configuration 1 as the draw speed increases. The results forConfiguration 3, however, show that the increase in fiber attenuationcan be compensated at the higher draw speed of Configuration 2 byoperating slow cooling device 220 with a graded ambient temperatureprofile.

To validate results from the model, a series of optical fibers wasprepared in an experimental fiber draw process using the fiber designand processing conditions corresponding to Configurations 1, 2, and 3.For each configuration, a series of several fibers was drawn andattenuation at 1310 nm was measured. For Configuration 1, theattenuation ranged from 0.3205 dB/km-0.3224 dB/km with a median value of0.3213 dB/km over the series of fibers. For Configuration 2, theattenuation ranged from 0.3200 dB/km-0.3211 dB/km with a median value of0.3208 dB/km over the series of fibers. For Configuration 3, theattenuation ranged from 0.3197 dB/km-0.3215 dB/km with a median value of0.3203 dB/km over the series of fibers. The experimental results areconsistent with predictions of the model.

The results of this example show that the graded ambient temperatureprofile associated with Configuration 3 leads to higher fibertemperatures over a much longer portion of slow cooling device 220 thanis observed for the constant ambient temperature profiles associatedwith Configurations 1 and 2 (compare trace 263 with traces 261 and 262).The fictive temperature of the fiber produced with the graded ambienttemperature profile of Configuration 3 is accordingly reduced relativeto the fictive temperatures of fibers produced with the constant ambienttemperature profiles of Configurations 1 and 2 (compare trace 266 withtraces 264 and 265). The fiber temperature at entrance 232 of fiber-turndevice 230, however, is the same for Configurations 1, 2, and 3. Theresults show that a slow cooling device configured with a graded ambienttemperature profile produces fibers having lower fictive temperaturethan corresponding slow cooling devices configured with a constantambient temperature profile, while maintaining the flexibility needed toestablish targeted temperatures downstream of the slow cooling devicewithout compromising fiber attenuation when operating at higher drawspeeds.

Example 2

In this example, the cooling rates in a slow cooling device are modeled.Cooling rate is defined as the rate of change of fiber temperature withrespect to time and is expressed in units of ° C./s. The slow coolingdevice is of the type shown in FIG. 2 and included a controlled coolingregion with six zones, each of which was 1 m in length along the processpathway. Cooling rates for Configuration 1 described in Example 1 andConfiguration 5 were determined. For Configuration 5, the ambienttemperature profile was graded. The ambient temperature in each zone waskept constant, but different constant ambient temperatures were used insome of the zones. Specifically, the ambient temperatures of zones 1-3were set at 1165° C., the temperature of zone 4 was set at 875° C., andthe temperatures of zones 5 and 6 were set at 725° C. The ambienttemperature profiles of Configurations 1 and 5 are shown as dotted linesin FIG. 5 and are associated with the temperature scale shown at theright abscissa. Traces 271 and 273 depict the ambient temperatureprofile of Configurations 1 and 5, respectively. Distance in FIG. 5corresponds to distance from the entrance of the controlled coolingregion of the slow cooling device and extends from 0 cm (entrance of thecontrolled cooling region) to 600 cm (exit of the controlled coolingregion). The entrance to the controlled cooling region of the slowcooling device corresponds to the entrance to zone 1 and the exit of thecontrolled cooling region of the slow cooling device corresponds to theexit from zone 6. The draw speed for both Configurations 1 and 5 was 42m/s.

FIG. 5 also shows cooling rates of the fiber temperature as a functionof position in the controlled cooling region of the slow cooling device.Traces 275 and 277 show cooling rates for Configurations 1 and 5,respectively, where cooling rates are shown on the scale at the leftabscissa. For Configuration 1, the cooling rate shows a monotonicdecrease from about 4400° C./s near the entrance of the slow coolingdevice to about 1300° C./s near the exit of the slow cooling device. Thecooling rate for Configuration 5, in contrast, is non-monotonic andshows discontinuities or step changes at boundaries between zonesmaintained at different constant ambient temperatures. The cooling ratewithin each zone and across zones maintained at the same constanttemperature is monotonically decreasing, but the overall cooling rate isnon-monotonic due to differences in zone temperatures along the processpathway through the slow cooling device. Relative to Configuration 1,slower cooling rates are observed for Configuration 5 in the upstreamportion of the slow cooling device (zones 1-3) due to the higher ambienttemperature. As noted above, slow cooling rates are conducive tostructural relaxation of the glass and a reduction in fiber fictivetemperature. Because of the slower cooling rate, the fiber temperatureat the exit of zone 3 is higher for Configuration 5 than forConfiguration 1. By lowering the ambient temperature in the downstreamportion of the slow cooling device (zones 4-6) relative to Configuration1, the cooling rate for Configuration 5 is higher than the cooling rateobserved for Configuration 1. The higher cooling rate for Configuration5 in the downstream portion of the slow cooling device permitsadjustment of the fiber temperature of Configuration 5 at the exit ofthe slow cooling device. For a given target fiber exit temperature,Configuration 5 provides an optical fiber having a lower fictivetemperature than Configuration 1.

Unless otherwise expressly stated, it is in no way intended that anymethod set forth herein be construed as requiring that its steps beperformed in a specific order. Accordingly, where a method claim doesnot actually recite an order to be followed by its steps or it is nototherwise specifically stated in the claims or descriptions that thesteps are to be limited to a specific order, it is no way intended thatany particular order be inferred.

It will be apparent to those skilled in the art that variousmodifications and variations can be made without departing from thespirit or scope of the illustrated embodiments. Since modifications,combinations, sub-combinations and variations of the disclosedembodiments that incorporate the spirit and substance of the illustratedembodiments may occur to persons skilled in the art, the descriptionshould be construed to include everything within the scope of theappended claims and their equivalents.

What is claimed is:
 1. An apparatus for processing an optical fibercomprising: a slow cooling device, said slow cooling device having anentrance for receiving an optical fiber, an exit for delivering anoptical fiber, and a controlled cooling region between said entrance andsaid exit, said controlled cooling including two or more zones forprocessing an optical fiber, said two or more zones including a firstzone maintained at a first average ambient temperature and a second zonemaintained at a second average ambient temperature, said first zonebeing upstream of said second zone, said second average ambienttemperature being at least 500° C., said first average ambienttemperature being greater than said second average ambient temperatureby at least 100° C.
 2. The apparatus of claim 1, wherein said firstaverage ambient temperature is at least 1100° C.
 3. The apparatus ofclaim 1, wherein said second average ambient temperature is at least900° C.
 4. The apparatus of claim 1, wherein said first average ambienttemperature is greater than said second average ambient temperature byat least 500° C.
 5. The apparatus of claim 1, further comprising a thirdzone, said second zone being upstream of said third zone, said thirdzone having a third average ambient temperature, said third averageambient temperature being at least 100° C. greater than said secondaverage ambient temperature.